This invention relates to a jet propulsion unit for a watercraft and more particularly to an improved compact jet pump and prime mover therefor.
The use of jet propulsion units for watercraft is widely accepted and continues to expand. With these types of units a prime mover, normally an internal combustion engine, powers a jet pump which draws water from the body of water in which the watercraft is operating and discharges it through a discharge nozzle for providing a propulsion force for the watercraft. This type of propulsion unit has a number of advantages in that it permits operation in relatively shallow water, provides a neat and compact assembly and has other advantages.
With the conventional type of construction the jet pump has an impeller shaft that extends generally forwardly, frequently through the water inlet opening of the jet pump. The impeller shaft is coupled to the output shaft of the prime mover for driving it. Normally the prime mover is positioned at the front of the jet pump and thus the jet pump and the prime mover are in longitudinally aligned positions. Frequently the prime mover will be disposed forwardly of the transom and the jet pump will be disposed at least in part rearwardly of the transom.
There are, however, many instances when it is desirable to provide the jet pump also forwardly of the transom. This is done normally by mounting the jet pump at least in part in a tunnel formed at the center and to the rear of the hull. Such an arrangement provides a very neat overall configuration of the watercraft and also reduces its effective length. However, where the engine is disposed forwardly of the jet pump then the engine, jet pump combination can intrude significantly into the cockpit area of the watercraft.
It is, therefore, a principle object of this invention to provide an improved and compact jet propulsion unit and prime mover unit for a watercraft.
It is a further object of this invention to provide an improved jet propulsion unit and prime mover wherein the prime mover and jet propulsion unit are disposed in a side-by-side or top-to-bottom relationship so as to reduce the overall length of the assembly.
One problem with jet propulsion units is that the maximum permissible speed of the impeller is somewhat limited. The reason for this is that if the impeller is driven at too high a speed, then, cavitation can result and the pump efficiency deteriorates.
On the other hand, in order to provide a compact propulsion unit, particularly if internal combustion engines are employed, it is desirable to operate the engine at a high speed to achieve a high power output from a low displacement. This results in an incompatibility of the speed of the engine output shaft and the impeller shaft.
It is, therefore, a still further object of this invention to provide an improved jet propulsion unit and prime mover therefor, that incorporates a compact construction and also a transmission that permits the engine output shaft to be run at a higher speed than the impeller shaft.